Monster or Area levels and experience?

I am still doing the storyline. Sometimes I breeze through an area, and the monsters are blue/green colored, indicating their level is similar to mine or slightly lower (right?), with the occasional orange-red, which is nice for a change. Then I will enter some side area, or connecting area, and the monsters all have significantly higher level than mine. Sometimes I realize I forgot a quest, and go back to an area, all the monsters are way underleveled, and I find the side area I was looking for, and suddenly the monsters are my level or higher again.
Q1: Do some areas spawn monsters based on player level? How can I find out which ones? Or is it: Area level = monster level +/- 3 or so?
Q2: I see people talking about “farming for experience” – do you get more or less experience for monsters below or above your level? Do I always have to find monsters at my level to avoid penalties (like in PoE)?
Q3: Or when people mean “Farm”, do they only mean farm for items? In which case: Item drop level = area level or monster level? Or player level? Or is there an interconnection (see question 1)?

thanks tons

There are certain areas that are not meant to be visited until you are higher level. Usually, these are barred by barriers you need dynamite (or something else) to open. Places like the Depraved Sanctuary (under Burrwitch), the East Marhs (from Mud Row), the Sunken Reliquary (from Forgotten Passage), the Immolation (from Deadman’s Gulch).

Q1: If you check the wiki, you will see the normal “lower range” level of monsters here is higher usually then what level you are when you get there:
Anywhere else, monster levels are the same as yours, +/- 3 or so. Of course, monster levels are capped on Normal/Elite difficulty.
Q2: If the number of the monster’s level is blue when you mouse-over it, that means it will give less experience, and Heroes/Bosses of Blue color will not drop any Rare components when killed. They can and will still drop Elite/Legendary items, but it will be matching their level, not yours.
Q3: Honestly I don’t see why you should farm for XP. With the expansion, I completed the game with my Blademaster on Normal/veteran, and was 70th level when I finished. Completing the game on Elite and Ultimate would definitely get you up to 100th level.

A1: yes especially in Normal and Elite there are mobs level arrangement, for example Lower crossing area in Normal the level is about 1-7, for higher level you have the level of mobs scales too but not higher than 7 (heroes and bosses always have +2-10 levels higher than trash mobs), in Ultimate the level cap of mobs removes as they always have equal or higher level than you.
A2: as long as the level diffirent between you and mobs not higher than 9.
A3: farming is good for high level as you dont need a good gear to complete Normal and Elite, the mobs (focus on heroes and bosses as trash mobs rarely drop good things) with level 50+ may drop legendaries, 75+ drop better legendaries, and 94+ may drop the mythical version.

For me, even 80+ enemies dropped Mythical Epics/Legendaries, or blueprints. I think that’s the cap for those, not 94+
And regular enemies as well as chests/containers can easily drop purple or blue stuff too, so it is worth checking those, naturally chances are higher with the Treasure Troves and Boss stashes.